Clear and simple makes things easy, but easy is not always better. Also, the “life begins at conception” position only seems clear on the surface, but if you look deep enough things get quite muddled.

For example, is a zygote a single person? What if it later divides and becomes twins or triplets: did the twin’s life begin at conception? Did one life become two? Is a zygote a ball of life that can become one or more people?

What about miscarriages? It’s thought as many as half of all pregnancies end in miscarriage, but most happen so early that the carrier is not even aware they’re pregnant. If you come across a family with four kids, do you assume they likely had another 3-4 lost lives via miscarriage and hold a funeral for them?

Should people start getting child tax benefits as soon as they have a positive pregnancy test? Or is “life starts at conception” only relevant when we’re talking about abortion, but conveniently ignored everywhere else?

And what if there is a complication with pregnancy, where if an abortion is not performed both the carrier and developing human will likely die, but if an abortion is performed only the developing human will likely die? Is it now permissible? What if the carrier is a 14 year old who was raped, is suicidal, and has a high chance of stabbing themselves in the abdomen to try to self-abort if they’re not able to get an abortion: should they be restrained in a padded room until the baby is born, forced to serve as an incubator for a baby that the state will then take?

Even when your cutoff is strict, it is not always “clear” because this is a complex issue without a clear answer.

But to answer your question specifically:

Pro-choice people generally recognize that abortion is not desirable, but disagree exactly what the rules should be. Abortion does the least harm when the pregnancy is a single cell (zygote,) and in the embryo stage where most abortions occur the developing human is essentially a collection of multiple cell lines becoming differentiated into tissue but not yet developing functional organs (you’ll often hear this called “a clump of cells.”)

As the embryo develops into a fetus, the heart and brain develop and start functioning, which is where some pro-choice people start to draw a line. Others point toward viability: at about 22 weeks, a few fetuses have been known to survive with extraordinary health measures. By 36 weeks, fetuses can be live born without any extra health issues from being born early. So starting about 20 weeks, we start to recognize that pregnancies become more and more viable: that’s where a lot of people draw the line.

A very small percentage of abortions are done late in pregnancy, typically for health reasons. Not all pro-choice people are in favor of legalizing this, but many feel that in these situations, abortion is a tough decision that is best made by a patient in a careful discussion with their doctor, not by a politician they will never meet. So while these pro-choice people may not wish to see an abortion performed within a week or two of natural birth, they do not want to outlaw it so that the option is there for people who truly need it.